Adds v3 API extension discovery filtering

Adds ability for a v3 loaded extension to be visible in
/v3/extensions to be dependent on the discoverable action policy
for that extension. Note that this does not actually effect whether
or not the functionality provided by the extension is accessible

Implements blueprint nova-v3-api-filter

Change-Id: I3f6ba7ea59e7abfa9d57b79ab18aa5b675e64118
port agent API into v3 part1

This changeset only copies the v2 files into the appropriate v3
directories unchanged. The copy as-is will not be loaded by either
the v2 or v3 extension loaders. The second changeset will then
make the changes required for it to work as a v3 extension.

This is being tried in order to make reviewing of extension
porting easier as gerrit will display only what is actually
changed for v3 rather than entirely new files.

Partially implements: bp v3-api-extension-versioning

Change-Id: Ib7c489769d9b07797366be5920cc23ab2074a768
Merge "Adds v3 API extension discovery filtering"
Merge "port agent API into v3 part1"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp